Huayao Zou

Control of differential tectonic evolution on petroleum occurrence in Bohai Bay Basin

The Bohai Bay Basin (BBB) is the most petroliferous Cenozoic basin in the east of China. It consists of seven depressions. Each depression has been subjected to different stress states and then has experienced varying faulting processes since the Neogene, especially during the Neotectonism (from the Pliocene to the present). On the basis of the investigation of fault patterns, fault densities and fault activity rates (FARs) for each depression, this paper demonstrates the discrepancy of faulting development and evolution across the BBB. The dynamic mechanism for the differences in faulting is also discussed by the analysis of the regional stress state. The Bozhong Depression is just situated in the transtensional zone induced by the two active strike-slip faults, namely Yingkou-Weifang and Beijing-Penglai. In this depression, the major faults which cut through the Paleogene or the Cenozoic have had higher than 10 m/Ma FARs since the Neogene, and the highest FARs have reached or exceeded 25 m/Ma during the Neotectonism. As a result, most of the petroleum has migrated along these major faults and accumulated within the Neogene. In contrast, in the other depressions of the BBB away from the Bozhong Depression, the FARs of the major faults were decreased to lower than 10 m/Ma since the Neogene, and tended to be zero during the Neotectonism. Therefore, the major faults could not serve as vertical conduits for petroleum migration, and the petroleum was entrapped in the Paleogene. Consequently, the faulting since the Neogene, especially during the Neotectonism, controlled the petroleum richness in vertical strata.

Heterogeneous distribution of pyrobitumen attributable to oil cracking and its effect on carbonate reservoirs: Feixianguan Formation in the Jiannan gas field, China

Few previous studies have focused on solid bitumen distribution and its effect on gas reservoir quality during oil cracking. Solid bitumen is commonly found in both gas and nongas reservoirs of the Triassic Feixianguan Formation in the Jiannan gas field. The natural gases are mainly secondary cracking gases of oil generated from source rock of the Permian Wujiaping Formation , and the reservoir experienced temperatures above 150°C (302°F) for about 35 m.y. A relatively narrow range of values and a wide range of values and widespread solid bitumen indicate that oil cracking took place in the gas field. Low concentrations of (commonly 8.0%) can be high-quality gas reservoirs after oil cracking and should be favorable targets for future gas exploration in the northeastern Sichuan Basin and adjacent areas.

Factors controlling petroleum accumulation and leakage in overpressured reservoirs

This paper reviews the hydrocarbon-retaining properties of overpressured reservoirs and discusses the mechanisms for petroleum accumulation, preservation and loss in overpressured reservoirs, and the factors controlling hydrocarbon column heights in overpressured traps. Four types of overpressured traps (filled, underfilled, unfilled, and drained) are recognized. The diversities in petroleum-bearing properties reflect the complexities of petroleum accumulation and leakage in overpressured reservoirs. Forced top seal fracturing, frictional failure along preexisting faults, and capillary leakage are the major mechanisms for petroleum loss from overpressured reservoirs. The hydrocarbon retention capacities of overpressured traps are controlled by three groups of factors: (1) factors related to minimum horizontal stress (tectonic extension or compression, stress regimes, and basin scale and localized pressure–stress coupling); (2) factors related to the magnitudes of water-phase pressure relative to seal fracture pressure (the depth to trap crest, vertical and/or lateral overpressure transfer, mechanisms of overpressure generation); and (3) factors related to the geomechanical properties of top seals or sealing faults (the tensile strength and brittleness of the seals, the natures and structures of fault zones). Commercial petroleum accumulations may be preserved in reservoirs with pressure coefficients greater than 2.0 and pore pressure/vertical stress ratios greater than 0.9 (up to 0.97). The widely quoted assumption that the fracture pressure is 80%–90% of the overburden pressure and hydrofracturing occurs when the pore pressure reaches 85% of the overburden pressure significantly underestimates the maximum sustainable overpressures, and thus, potentially the hydrocarbon-retention capacities, especially in deeply buried traps. Lateral and/or vertical water-phase overpressure transfer from deeper successions plays an important role in the formation of unfilled and drained overpressured traps. Traps in hydrocarbon generation-induced overpressured systems have greater exploration potential than traps in disequilibrium compaction-induced overpressured systems with similar overpressure magnitude.

Tracing migration pathways by integrated geological, geophysical, and geochemical data: A case study from the JX1-1 oil field, Bohai Bay Basin, China

Tracing petroleum migration pathways is essential for predicting petroleum occurrence and reducing exploration risks associated with hydrocarbon charge, but a difficult task because of rapid lateral and vertical facies changes in lacustrine basins. An integration of geological, geophysical, and geochemical analysis is employed to investigate the origin of crude oil, the carrier-bed architecture, and migration pathways from source to trap in the JX1-1 oil field, Liaodong Bay subbasin, Bohai Bay Basin. Detailed geochemical studies suggest that three potential source-rock intervals (, , and ) exist in the Liaodong Bay subbasin, and crude oil in the JX1-1 field was derived from the and source rocks. The carrier beds from and source rocks to the trap were characterized using geophysical data. The fan-delta sandstone in the Member has an immediate contact with source rock and served as dominant conduit for the expulsion and migration of oil generated from source rock. The braided-delta sandstones overlying the source rock served as dominant conduit for -sourced oil. The focusing of petroleum migration pathways and the merge of migration pathways in and sandstones account for the accumulation of the JX1-1 field and the mixing of - and -sourced oil in the field. This study suggests that the distribution of permeable sandstones and their stratigraphic contact with the source rocks are key for petroleum migration and occurrence, and integration of geophysical, geological, and geochemical studies provide an effective way to trace petroleum migration pathways.

Tan-Lu fault system and its significance in oil accumulation in the central Liaodong Bay subbasin, Bohai Bay Basin, China

The Tan-Lu fault system in the central Liaodong Bay subbasin, Bohai Bay Basin, has complex structural characteristics, and its tectonism during the Cenozoic is an important factor in oil accumulation. Three-dimensional seismic data were used to document the structural features and evolution of the system. Variations in source rock occurrence, oil catchment area, and faulting intensity were comprehensively evaluated to discuss the heterogeneity of oil occurrence along the system. Detailed analysis of the seismic data indicates a right-lateral slip for the Tan-Lu fault system. Transpression and transtension occur on two branches of the Tan-Lu fault with different orientations, indicating a likely slip azimuth that is between the two orientations, i.e., 30°–35°. The strike-slip began at the middle stage of deposition of the Shahejie Formation and reached its climax during deposition of the first and second members of the Dongying Formation. To date, the slip is still continuing. In the transpressional system, the lower strata on opposite sides of the strike-slip fault were shortened and folded, whereby the upper strata were stretched and normal faulted. As a result, a negative flower structure developed immediately above a positive flower structure. Strain distribution in this system is similar to that in a classic fold. Moderate tectonic deformation could enlarge the oil catchment area, benefiting oil accumulation. Weak or intense deformation is unfavorable for commercial oil accumulation because of a small oil catchment area and poor oil preservation, respectively.

Petroleum charge and entrapment along active faults: Study of the accumulation mechanism of the Qinhuangdao 29 oil field on the slope of the Shijiutuo uplift, Bohai Sea

In the Qinhuangdao 29 (QHD29) oil field, oil generated from the first member of the Shahejie source rocks is mainly contained in Paleogene reservoirs, whereas deeper oil sourced from the third member of the Shahejie (Es3) source rocks is generally accumulated in Neogene reservoirs. The present study was undertaken to better understand the differences in petroleum accumulation in the QHD29 oil field and to provide suggestions for future petroleum exploration on the Shijiutuo uplift. Laser Raman spectroscopy reveals that carbon dioxide (CO2) exists with hydrocarbon gas in the same fluid inclusions. Measured homogenization temperatures of aqueous inclusions range from 80°C to 160°C (176°F to 320°F), indicating late-stage rapid petroleum accumulation with a charging time of no earlier than 5.1 Ma. The results of grains containing oil inclusions measurements reveal the presence of paleo-oil accumulation in the current natural gas column. In terms of the boundary fault activity rate (FAR), CO2 distribution is quite relevant to the late-stage activity of boundary faults, with high content of CO2 corresponding to the section of the F1 fault with high-intensity activity (higher FAR values). Both Neogene and Paleogene reservoirs in well A1 contained predominantly Es3-derived oil and were accompanied by abundant mantle CO2. This reveals the segmented vertical transport of petroleum in the fault: both the mantle-derived CO2 and hydrocarbons vertically migrated and accumulated in shallower reservoirs in the high-activity intensity section of the boundary fault. This may account for the occurrence of predominantly Es3-derived oil in the reservoirs near the section of the fault with high activity intensity. In the eastern part of the QHD29 oil field, vertical migration may have been limited because of the relatively low intensity of fault activity, and the distribution of sandstones seems to dominate the petroleum accumulation. Our research reveals that lithologic traps in the Es3 stratum may still have great potential for exploration along the slope of the Shijiutuo uplift.

Thermal evolution and applications of aromatic hydrocarbons in highly mature coal-bearing source rocks of the Upper Triassic Xujiahe Formation in the northern Sichuan Basin

Based on the GC-MS analytical data of aromatic fractions of over forty highly mature coal-bearing source rock samples collected from the Upper Triassic Xujiahe Formation in the northern Sichuan Basin, the thermal evolution of aromatic hydrocarbons during late-mature to over-mature stage (Ro=1.13%–2.85%) was characterized, and aromatic indicators suitable for recognizing the organic source and sedimentary environment of high maturity source rocks were discussed. The results indicated that the concentrations of low carbon-cycle naphthalene as well phenanthrene series reduce gradually with increasing Ro at the highly mature levels. However, some high-cyclic components such as chrysene, benzofluoranthene, and benzo[e]pyrene are relatively enriched, in companying an enhancement of parent aromatic compounds. The variations are attributed to thermal cracking and polymerization reactions due to continuous dehydrogenation under enhanced burial temperature. As thermal maturity rises, MPI1 (Methylphenanthrene Index) values display a two-modal varying trend, namely, increasing when Ro is below 1.80% and decreasing above 1.8% Ro. The relationships between Ro and MPI1 are Ro=0.98MPI1+0.37 for Ro<1.80% and Ro=−0.90MPI1+3.02 at Ro>1.8%, being different from the previous research. The amount of dibenzofurans declines sharply at Ro higher than 1.1%, leading to a significant change of relative composition among dibenzothiophenes, dibenzofurans and fluorenes (referred as three-fluorenes series composition). Thus, this parameter appears to be unsuitable for identifying the sedimentary environment of the highly matured source rocks. 4-/1-MDBT (methyldibenzothiophene) ratio could be served as an effective indicator for organic facies, and can distinguish coals from mudstones at over-maturity in this case. The ratios of 2,6-/2,10-DMP (dimethylphenanthrene) and 1,7-/1,9-DMP and the relative abundance of triaromatic steroids in these highly mature rocks could be considered as biological source parameters for relative input of terrigenous versus aquatic organic matter.

Origin of Taiwan Canyon and its effects on deepwater sediment

The continental slope of the Taiwan Shoal, which has cultivated numerous submarine canyons, is located in a passive continental margin environment. However, the trend of the Taiwan Canyon, with its 45° intersection angle, is obviously different from that of the erosion valley downward along the continental slope. A distinct break is present in the lower segment of the Taiwan Canyon, which then extends from west to east parallel to the continental slope until finally joining the Manila Trench. By utilizing multiple-beam water depth data, high-resolution seismic data, and sediment cores, this study describes the topographic characteristics of the Taiwan Canyon and provides a preliminary discussion on the origin of the Taiwan Canyon and its effect on deepwater sediment. The terrain, landform, and sediment of the Taiwan Canyon exhibit segmentation characteristics. The upper segment is characterized primarily by erosion, downward cutting with a V shape, and wide development of sliding, slumping, and other gravity flow types. The middle segment is characterized mostly by U-shaped erosion-sedimentation transition and development of an inner levee. The lower segment is characterized primarily by sedimentation and development of a sediment wave. The bottom current has a significant reworking effect on the interior sediments of the canyon and forms reworked sands. The formation and evolution of the Taiwan Canyon is closely related to sediment supply, gravity sliding (slumping), faulting activities, and submarine impaling. Given the sufficient terrigenous clastic supply, the sediments along the continental shelf edge continuously proceed seaward; gliding and slumping in the front edge provide driving forces for the formation of the canyon. Faulting activities result in stratum crushing, and the gravity flow takes priority in eroding the relatively fragile stratum. Thus, the direction of the extension of the canyon crosses the surrounding erosion valley obliquely. Seamounts are formed through submarine impaling. Owing to seamount blocking, the lower segment of the canyon is turned toward the east-west direction. Large amounts of sediments overflow at the turning, forming sediment waves.

Distribution, Controlling Factors and Oil-source correlation of Biodegraded Oils in the Bohai Offshore Area, Bohai Bay Basin, China

The vast majority of discovered oils in the Bohai offshore area have undergone biodegradation varying from 1 to 9 on the PM scale (a scale to rank the level of biodegradation proposed by Peters and Moldowan, 1993). The distribution and biodegradation extent of all discovered oils in the Bohai offshore area were investigated systematically using geologic and geochemical data to reveal controlling factors of varying levels of biodegraded oils. Based on the analysis on environment and maternal significances and the resistance to degradation of biomarkers, the biomarker parameter assemblage that is suitable for the oil-source correlation of severely biodegraded oils (higher than PM 6) in the Bohai offshore area was determined. Spatial distribution and biodegradation extent are mainly controlled by current burial depth, the duration of biodegradation, the area of the oil-water-contact (OWC) and a late strike-slip movement of the Tanlu fault. Almost all biodegraded oils are found in shallow reservoirs above 2000 m. The longer the oils are present in these reservoirs or the larger the area of OWC the reservoirs show, the greater the extent of biodegradation will be. The late strong strike-slip movement of the Tanlu fault may have significantly enhanced the biodegradation extent of several oils in fields located in the Tanlu fault zone by introducing oxygenated freshwater from the surface or near-surface and creating a more suitable environment for biodegradation. C19 tricyclic terpane/C23 tricyclic terpane (C19/C23 TT), C24 tetracyclic/C26 tricyclic terpane (C24 Tet/C26 TT) and Gammacerane/C24 Tetracyclic (Gam/C24 Tet) do not seem to be influenced by biodegradation and show obvious differences between the three different source rock intervals. Such a biomarker parameter assemblage can be used successfully to determined the origin of severely biodegraded oils (higher than PM 6) by correlating with extracts of possible source rocks in the Bohai offshore area.